ReactJS is an open-source JavaScript library developed by Facebook. It is used for building user interfaces (UIs) for single-page web applications and mobile applications.

React allows developers to build UI components that can be reused across different parts of an application, making it a popular choice for building large-scale web applications.

Some key features of React include its use of a virtual DOM (Document Object Model), which allows it to efficiently update and render changes to the UI without reloading the entire page. React also uses a declarative programming style, which makes it easier to reason about and debug code.

React has a large and active community of developers, and there are many third-party libraries and tools available for working with it, such as Redux for managing application state, React Router for client-side routing, and Next.js for server-side rendering.

Overall, React has become a popular choice for building modern web applications due to its performance, flexibility, and ease of use.